Ink pen for dispensing ink having time-dependent characteristics

ABSTRACT

A pen includes a housing; a first reservoir inside the housing and configured to contain a first fluid, the first reservoir including a first opening configured to dispense the first fluid; a second reservoir inside the housing and configured to contain a second fluid, the second reservoir including a second opening configured to dispense the second fluid; a first writing tip extending from the housing and configured to dispense a substantially homogeneously mixed fluid to a writing surface; and a mixing region connected to the first and second openings and the first writing tip and configured to substantially homogeneously mix the first and second fluids and dispense the substantially homogeneously mixed fluid to the first writing tip.

BACKGROUND

There are many situations in which there is a need to determine the timeand/or date of a writing. Possibly the most obvious example to anyonereading this document is the need to prove the date of a writing to showconception of an invention on or before the effective date of a priorart document or event. For example, in the U.S. first-to-invent system,a first inventor of an invention who files a U.S. patent applicationafter a second inventor files a U.S. patent application on the samesubject matter may be able to “win” the patent rights in a prioritycontest if the first inventor can show prior conception and diligencefrom before the second inventor's conception to the first inventor'sU.S. filing date. One way to show prior conception is, e.g., to providedetailed laboratory notes that disclose the invention. Typically, toconvince a jury, the notes must be dated and witnessed.

A problem with this method is multifaceted. First, an independentinventor may be unfamiliar with U.S. law and may have insufficientcorroborating evidence to prove the asserted date of conception. Second,if the witness is biased (such as if the witness works for the samecompany as the inventor), her testimony may be insufficient tocorroborate the written lab notes. Third, if the witness is not one ofskill in the art, the witness may not be able to convince a jury thatshe knew the inventor possessed the invention on the asserted conceptiondate. If, in addition to any witness testimony, the inventor canphysically prove a date that the ink was placed on the lab note paper,this additional evidence may be sufficient to convince a jury of theasserted conception date. Thus, there has been a need to determine theage of ink. Of course, this need exists in other fields besidesinventorship, conception, and patents. For example, a person may want toprove that he signed a legal document on a particular date, or may wantto prove that he wrote certain entries in a legal document on differentdates. Such a need often arises in expensive litigation, such asinsurance litigation. A claimant, who may have been accused of insurancefraud by making several dated entries (dated on different days) on thesame day. The claimant may be able to counter this accusation bydetermining the age of the ink on each of the entries. One skilled inthe art will recognize that there is a wide host of situations in whichone needs to determine the age of ink deposited on a writing surface(such as paper).

Presently, ink dating analysis is based on the observation that typicalball-point ink does not fully dry for up to five years. Because wet inkand dry ink dissolve in a solvent at different rates, the approximateage of ink on a writing surface may be determined by dissolving a knownquantity of the drying ink in a solvent and timing the rate ofdissolution; the faster the dissolution, the more recently the ink wasapplied to the writing surface. There are a host of problems with thismethod. First, because currently available inks vary widely in theirproperties, such as chemical content, viscosity, and vapor pressure, theage of the ink determined by this method is very approximate, and cannotyield the kind of accurate dating required in some litigation. Second,the method is easy to fool. A defrauder may attempt to make a writtenink appear much older than it actually is simply by heating or cookingthe document. Applying heat to ink increases its vapor pressure and therate at which it dries. Alternatively or in addition, the defrauder mayplace the document in a vacuum chamber, where the ink will naturally drymore quickly than in the atmosphere.

One attempt to solve the first problem above (the inherent uncertaintiesin current ink aging analyses) is disclosed in U.S. Pat. Nos. 5,600,443and 5,759,246 to Frey et al. Frey discloses a method and ink compositionof two possible mechanisms. The first mechanism relates to detectingcolor shifts of pH sensitive compounds in the ink as the result of theevaporation of certain acidic or basic compounds thereof. The secondmechanism relates to the oxidation of compounds in the ink by reactionwith oxygen in the air. Frey does not solve the second problem posedabove (the ease of defrauding). For example, in the first mechanism, theevaporation of the certain acidic or basic compounds in the ink can beartificially accelerated by a defrauder by either heating the inkeddocument or placing the inked document in a vacuum chamber. In thesecond mechanism, the oxidation of the compounds can be artificiallyaccelerated by a defrauder by either heating the inked document orplacing the inked document in a pressure chamber in which the oxygendensity is higher than in the atmosphere. For example, because thesea-level atmosphere has a pressure of about 1 atm and contains about20% oxygen, the oxidation of the compounds may be artificiallyaccelerated by placing the inked document in a pressure chamber suchthat the pressure chamber contains a pressure higher than atmospheric,an oxygen content of greater than 20%, or both. Thus, it is relativelyeasy to make the ink of Frey appear that it was placed on the documentin question on a date much earlier than it actually was.

SUMMARY OF THE INVENTION

The present invention aims to solve one or more of these and otherproblems.

In a preferred embodiment of the present invention, a pen may comprise:a housing; a first reservoir inside the housing and configured tocontain a first fluid, the first reservoir comprising a first openingconfigured to dispense the first fluid; a second reservoir inside thehousing and configured to contain a second fluid, the second reservoircomprising a second opening configured to dispense the second fluid; afirst writing tip extending from the housing and configured to dispensea substantially homogeneously mixed fluid to a writing surface; and amixing region connected to the first and second openings and the firstwriting tip and configured to substantially homogeneously mix the firstand second fluids and dispense the substantially homogeneously mixedfluid to the first writing tip. A maximum length of the pen may not begreater than approximately ten inches. The pen may further comprise asecond writing tip extendable from the housing and configured to receiveonly the first fluid and to dispense the first fluid unmixed with thesecond fluid to the writing surface.

In a preferred aspect, the mixing region may have a length substantiallyparallel to and a width substantially perpendicular to an average flowdirection of the first and second fluids through the mixing region,wherein the length is at least twice the width. The mixing region maycomprise a porous material, such as felt. The mixing region and firstwriting tip may be configured to be removable from the pen and readilyreplaceable.

In another preferred aspect, the first opening may be substantiallygreater than the second opening. Further, the substantiallyhomogeneously mixed fluid may comprise substantially more of the firstfluid than the second fluid. Further, the first fluid may besubstantially more viscous than the second fluid.

In another preferred aspect, the first fluid may comprise a firstchemical, such as a resin, and the second fluid may comprise a secondchemical, such as a hardener, that is chemically reactive with the firstchemical. Further, the first and second chemicals may solidify into asolid product upon chemically reacting. The chemical reaction betweenthe first and second chemicals may have a known time dependence, anextent of chemical reaction between the first and second chemicals maybe indicative of a time of mixing of the first and second fluids.Further, at least one of the first and second chemicals may bedissolvable in a solvent, and the solid product may be substantiallyinsoluble in the solvent, and an extent of dissolution of the at leastone of the first and second chemicals in a fixed time may be indicativeof a time of mixing of the first and second fluids. Further, the solidproduct may decompose at a known rate, and an extent of decomposition ofthe solid product may be indicative of a time of dispensing of thesubstantially homogeneously mixed fluid to the writing surface.

In another preferred aspect, the first fluid may further comprise afirst dye, and the second fluid may further comprise a second dye,wherein a color of the substantially homogeneously mixed fluid isindicative of a concentration of the first fluid in the substantiallyhomogeneously mixed fluid.

In another preferred aspect, the first and second chemicals may solidifyinto a solid product at a rate such that a viscosity of thesubstantially homogeneously mixed fluid at 25 degrees Celsius is greaterthan 10,000 centipoise at any time after seven days after a time ofdispensing of the substantially homogeneously mixed fluid to the writingsurface, and the viscosity is less than 10,000 centipoise at any timebefore one day after the time of dispensing of the substantiallyhomogeneously mixed fluid to the writing surface.

In another preferred aspect, the first fluid may comprise a thirdchemical chemically reactive with a paper writing surface, and an extentof chemical reaction between the third chemical and the paper writingsurface may be indicative of a time of mixing of the first and secondfluids.

In another preferred aspect, the substantially homogeneously mixed fluidmay comprise a heat-activated indicator. The heat activated indicatormay comprise at least two reactants which at normal room temperature donot react with each other, but which, upon application of thermalenergy, undergo a chemical reaction with each other which results inchange of color.

In another preferred aspect, the first fluid may further comprise athird chemical and the second fluid may comprise a fourth chemicalchemically reactive with the third chemical, and the chemical reactionbetween the third and fourth chemicals has a known time dependence, andan extent of chemical reaction between the third and fourth chemicalsmay be indicative of a time of mixing of the first and second fluids. Atleast one of the third and fourth chemicals may have a color, and thecolor changes during chemical reaction between the third and fourthchemicals, and a color of the substantially homogeneously mixed fluid isindicative of a time of mixing of the first and second fluids.

In another preferred aspect, the first fluid may further comprise afirst chemical identifier, such as at least a first rare-earth element,such that a relative concentration of the first rare-earth element inthe fluid ink is indicative of a manufacturer of the fluid ink. Thesecond fluid may further comprise a second chemical identifiercomprising at least a second rare-earth element, and a relativeconcentration of the first and second rare-earth elements in thesubstantially homogeneously mixed fluid may be indicative of aconcentration of the first fluid in the substantially homogeneouslymixed fluid.

In another preferred embodiment of the present invention, an ink pen fordispensing ink having time-dependent characteristics may comprise: awriting pen, comprising at least one reservoir configured to contain afluid, and further comprising a writing tip configured to dispense thefluid to a writing surface; and a fluid ink having time-dependentcharacteristics contained in the at least one reservoir, the inkcomprising: a first chemical, such as a resin; a second chemical, suchas a hardener; and a dye, wherein the first and second chemicals arechemically reactive and solidify into a solid product upon chemicallyreacting, wherein the writing pen and fluid ink are configured so that:at least the first and second chemicals are substantially chemicallyisolated from each other prior to dispensing the ink to the writingsurface; and at least the first and second chemicals are substantiallymixed with each other upon dispensing the fluid ink to the writingsurface so as to chemically react with each other. A maximum length ofthe pen may not be greater than approximately ten inches.

In a preferred aspect, the fluid ink may further comprise a thirdchemical chemically reactive with a paper writing surface, wherein anextent of chemical reaction between the third chemical and the paperwriting surface is indicative of a time of dispensing of the fluid ink.

In another preferred aspect, the fluid ink may be contained in exactlyone reservoir, and at least one of the first and second chemicals may beencapsulated in microcapsules. The microcapsules may have a diameter ofnot more than approximately 20 micrometers, the microcapsules eachhaving an outer wall that is substantially nonreactive with the firstand second chemicals at normal room temperature. Further, the writingtip may comprise a ball point configured to crush the microcapsules, andto release the one of the first and second chemicals into the other ofthe first and second chemicals, upon dispensing the fluid ink to thewriting surface.

In another preferred aspect, the chemical reaction between the first andsecond chemicals may have a known time dependence, and an extent ofchemical reaction between the first and second chemicals may beindicative of a time of dispensing of the fluid ink to the writingsurface. Further, at least one of the first and second chemicals may besoluble in a solvent and the solid product may be substantiallyinsoluble in the solvent, and an extent of dissolution of the at leastone of the first and second chemicals in a fixed time may be indicativeof a time of dispensing of the fluid ink to the writing surface.Further, the solid product may decompose at a known rate, and an extentof decomposition of the solid product may be indicative of a time ofdispensing of the fluid ink to the writing surface.

In another preferred aspect, the fluid ink may further comprise aheat-activated indicator and/or a chemical identifier.

In another preferred aspect, the fluid ink may further comprise at leasta third chemical chemically reactive with the dye such that chemicalreaction results in change of color, and the writing pen and fluid inkmay be configured so that: at least the dye and the third chemical aresubstantially chemically isolated from each other prior to dispensingthe ink to the writing surface; and at least the dye and the firstchemical are substantially mixed with each other upon dispensing thefluid ink to the writing surface so as to chemically react with eachother, and a color of the fluid ink may be indicative of an extent ofchemical reaction by the dye, and an extent of chemical reaction by thedye may be indicative of a time of dispensing of the fluid ink to thewriting surface. Further, the fluid ink may further comprise a fourthchemical chemically reactive with the third chemical, and a pH of acombination of the first and second chemicals may change upon reactionbetween the first and second chemicals, and a color of the dye may be pHsensitive. Further, the fluid ink may be contained in exactly onereservoir, and at least one of the dye and the third chemical may beencapsulated in microcapsules.

In another preferred aspect, the fluid ink may further comprise thirdand fourth chemicals chemically reactive with each other, and thewriting pen and fluid ink may be configured so that: at least the thirdand fourth chemicals are substantially chemically isolated from eachother prior to dispensing the ink to the writing surface; and at leastthe third and fourth chemicals are substantially mixed with each otherupon dispensing the fluid ink to the writing surface so as to chemicallyreact with each other, and an extent of chemical reaction between thethird and fourth chemicals may be indicative of a time of dispensing ofthe fluid ink to the writing surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side cross-sectional view of an ink pen according to apreferred embodiment.

FIG. 2 a shows an enlargement of the region indicated in FIG. 1.

FIG. 2 b shows an enlargement of the region indicated in FIG. 1, withthe mixing region 22 and first writing tip 24 removed.

FIG. 3 shows a side cross-sectional view of an ink pen according toanother preferred embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The disclosures of U.S. Pat. Nos. 3,389,125, 3,481,675, 3,634,276,3,887,287, 4,061,620, 4,425,178, 4,531,141, 4,833,226, 5,180,652,5,248,555, 5,306,092, 5,368,405, 5,368,905, 5,589,523, 5,600,443,5,718,513, 5,759,246, 6,030,118, 6,506,494, 6,579,829, and 6,602,594 arehereby incorporated by reference to the degree necessary to understandthe present invention and to enable one skilled in the art to make anduse the present invention.

Referring now to FIG. 1, a pen 2 includes a housing 4, first and secondreservoirs 6, 10 containing first and second fluids 8, 12. The firstreservoir 6 has openings 14, 16 configured to dispense the first fluid,and the second reservoir 10 has openings 18, 20 configured to dispensethe second fluid. The opening 16 of the first reservoir 6 is fluidlyconnected to a writing tip 26. For example, writing tip 26 may comprisea porous material, such as felt, which absorbs the first fluid 8 fromthe opening 16 by capillary action and dispenses the first fluid 8 to awriting surface substantially unmixed with the second fluid 12.Similarly, the opening 20 of the second reservoir 10 is fluidlyconnected to a writing tip 28. Writing tip 28 may comprise a porousmaterial, such as felt, which absorbs the second fluid 12 from theopening 20 by capillary action and dispenses the second fluid 12 to thewriting surface substantially unmixed with the first fluid 8. The pen 2also includes a mixing region 22 connected to a writing tip 24. Themixing region is connected to the first reservoir 6 via opening 14, andto the second reservoir 10 via opening 18. The mixing region maycomprise a porous material, such as felt, which absorbs the first fluid8 from the opening 14 and the second fluid 12 from the opening 18,substantially homogeneously mixes the fluids 8, 12, and dispenses thesubstantially homogeneously mixed fluid to the writing tip 24, which maythen dispense the substantially homogeneously mixed fluid to the writingsurface. The pen is preferably a writing pen, not longer thanapproximately ten inches, and preferably not longer than approximatelysix or seven inches.

The utility of the pen 2 is not limited to dispensing ink havingtime-dependent characteristics. For example, the first fluid 8 maycomprise a yellow ink or dye and the second fluid 12 may comprise a redink or dye. The substantially homogeneous mixture of the yellow and redink, as mixed and subsequently dispensed by the mixing region 22 to thewriting tip 24, is an orange ink, as understood by one skilled in theart. Thus, the present invention allows for three writing tips 24, 26,28, each dispensing a different colored ink, with the need to provideonly two different colored inks 8, 12 and corresponding reservoirs 6,10. Of course, a pen having three such reservoirs with different coloredinks could be capable of providing up to seven differently colored inksvia seven different writing tips. Such a variation is included in thescope of the present invention. Many other possible uses of pen 2 andcompositions of fluids 8, 12 will be apparent to one skilled in the art.

Referring now to FIG. 2 a, a close-up of the mixing region 22 of FIG. 1is shown. The mixing region 22 is configured to draw in fluids 8, 12from both reservoirs 6, 10 via respective openings 14, 18 and direct theflow of the fluids 8, 12 toward the writing tip 24, while in themeantime substantially homogeneously mixing the fluids 8, 12, such thatthe fluid dispensable from the writing tip 24 is substantiallyhomogeneously mixed. One means by which to ensure substantiallyhomogeneously mixing of the fluids 8, 12 is to provide a mixing region22 that is thin and long, so that the fluids 8, 12 have sufficient timeto mix via natural processes, such as diffusion. For example, as shownin FIG. 2 a, arrows 30 show an approximate flow path of the fluids 8, 12as they enter and flow through the mixing region 22. L is the length ofthe mixing region 22 substantially parallel to an average flow direction(i.e., the average of the approximate flow paths 30 of the fluids 8, 12)of the fluids 8, 12 through the mixing region 22, and W is the width ofthe mixing region 22 substantially perpendicular to the average flowdirection of the fluids 8, 12 through the mixing region 22. By making Lsubstantially greater than W, such as at least twice or thrice or fivetimes W, the fluids 8, 12 are in close enough contact for a long enoughtime to substantially homogeneously mix. Further, the mixing region 22may include a conical portion, such as in the writing tip 24, such thatthe fluids 8, 12 must pass through a very thin portion (i.e., W is verysmall) before being dispensed to the writing surface, to ensure propermixing.

FIG. 2 b is similar to FIG. 2 a, except that the mixing region 22 andwriting tip 24 have been removed. As will be discussed later, fluids 8,12 may contain chemicals that solidify upon chemically reacting witheach other. Thus, the substantially homogeneously mixed fluid inside themixing region 22 and writing tip 24 may, after time, become solid orhardened, thus preventing further use of the mixing region 22 andwriting tip 24. Thus, in a preferred embodiment, the mixing region 22and writing tip 24 are removable and readily replaceable with anothermixing region 22/writing tip 24. As shown in FIG. 2 b, fluids 8, 12enter mixing region 22 via through-holes 34, 32, respectively.Through-holes 32, 34 may comprise valves or porous materials, such thatfluids 8, 12 do not flow through through-holes 34, 32 unless and until amixing region 22/writing tip 24 is connected to them. The through-holes34, 32 may also serve as physical stops, so that a mixing region22/writing tip 24 may be inserted into the pen 2 (in an upwarddirection) until physically hitting through-holes 34, 32, after whichthe mixing region 22/writing tip 24 cannot move up further, and afterwhich a fluid connection is established between the first and secondreservoirs 6, 10 via openings 14, 18, respectively, and through-holes34, 32, respectively. As shown in FIGS. 2 a and 2 b, opening 18 may besubstantially greater than opening 14 (or vice versa), so that more ofthe second fluid 12 than the first fluid 8 flows into the mixing region22, so that the substantially homogeneously mixed fluid dispensable fromthe writing tip 24 comprises substantially more of the second fluid 12than the first fluid 8 (or vice versa). Alternatively, the first fluid 8may be substantially more viscous than the second fluid 12, so that,even where openings 14, 18 are approximately the same in size, more ofthe second fluid 12 than the first fluid 8 flows into the mixing region22.

Of course, mixing region 22 need not comprise a porous material. This isan example only. There are other ways, known in the art, tosubstantially homogeneously mix two fluids, preferably passively. Suchembodiments are included in the scope of the present invention.

In a preferred embodiment of the present invention, the first fluid 8comprises a first chemical and the second fluid 12 comprises a secondchemical chemically reactive with the first chemical. Either of thefirst and second fluids 8, 12 may also comprise a dye, so that theresulting substantially homogeneously mixed fluid dispensable from thewriting tip 24 serves as an ink providing a substantial contrast withthe writing surface, as understood by one skilled in the art. The firstand second chemicals may solidify into a solid product upon chemicallyreacting. For example, the first chemical could be a resin (i.e.,adhesive resin) and the second chemical could be a hardener or curingagent. Typically, an adhesive resin is one which, when admixed with ahardener or curing agent, will react at room temperature to form aninsoluble, thermoset product. Resins, hardeners, and epoxies, as well astheir specific chemical compositions, are well known in the art, andwill not be described in further detail here. They are described, forexample, in U.S. Pat. Nos. 3,389,125, 4,061,620, and 4,425,178.Heat-curable mixtures are known in the art, such as in U.S. Pat. No.3,634,276. Heat-curable epoxies and epoxies having very long pot lives(e.g., 6 months) are known in the art, such as in U.S. Pat. No.4,833,226.

The chemical reaction between the first and second chemicals may alsohave a known time dependence, such that an extent of chemical reactionbetween the two chemicals may indicate a time that the first and secondfluids were substantially homogeneously mixed in the mixing region 22.For example, the chemical reaction between the resin and hardener, suchas by polymeric crosslinking, in many cases takes a long time to “fully”complete; i.e., the crosslinking continues to occur long after theinitial mixing. Because the resin may be soluble in a common solvent,such as acetone or alcohol, and because the cured epoxy is typicallysubstantially insoluble in such a solvent, an extent of reaction betweenthe resin and hardener may be determined under laboratory conditions byplacing the cured epoxy in the solvent for a fixed period of time anddetermining a level of dissolution of the resin, as will be understoodby one skilled in the art. Based on the level or extent of dissolution,an approximate time of the initial mixing of the first and second fluids8, 12 may be determined. Further, the resulting cured epoxy maynaturally decompose with exposure to heat and/or radiation, as known tothose skilled in the art. If the rate of decomposition is approximatelyknown, then an extent of decomposition of the cured epoxy may indicatean approximate time of the initial mixing of the first and second fluids8, 12.

Preferably, the first and second chemicals, such as a resin andhardener, respectively, solidify into a solid product, such as a curedepoxy, at a rate such that a viscosity of the substantiallyhomogeneously mixed fluid at 25 degrees Celsius is greater than 10,000centipoise (preferably greater than 20,000 or 50,000 centipoise) at anytime after seven days after a time of mixing of the first and secondfluids 8, 12, and the viscosity is less than 10,000 centipoise at anytime before one day after the time of mixing of the first and secondfluids 8, 12. If the viscosity of an ink is much greater than 10,000centipoise, it is very difficult to use the ink as a ball-point ink oran ink for a felt-tip pen. Thus, by insuring that the first and secondfluids 8, 12 react upon mixing such that they cannot be dispensed as afluid ink after a certain period of time after mixing, then it is verydifficult for a defrauder to use the pen of the present invention tofake a document or writing. For example, if he knows that he intends tofraudulently write a document at some point in the future, he may mixfirst and second fluids 8, 12 now. However, in the present invention,because first and second fluids 8, 12 react to solidify into a solidproduct, the defrauder will not be able to dispense the resultingsubstantially homogeneously mixed fluid more than a week after mixing.Thus, with the proper choice of first and second chemicals in first andsecond fluids 8, 12, respectively (such as resin and hardener,respectively), the pen 2 will provide a certain “window” of opportunity,at the beginning of which the fluids 8, 12 are mixed, and at the end ofwhich the resulting substantially homogeneously mixed fluid is too hardor solid or viscous to be dispensed to a writing surface as a fluid.This window may be a few minutes, a few hours, a day, a few days, or aweek or more. However, while the approximate time at which the first andsecond fluids 8, 12 were mixed may be determined according to the pen,ink, and method of the present invention, it may be difficult orimpossible to determine exactly when in the window the substantiallyhomogeneously mixed fluid/ink was dispensed or deposited to the writingsurface. This is because the substantially homogeneously mixed fluidcould have been deposited anytime during the window.

For example, assume that the first fluid 8 comprises a resin and thesecond fluid 12 comprises a hardener and black dye, chosen such that theresin and hardener harden within 24 hours upon mixing to a point atwhich the mixture can no longer be dispensed to a writing surface as afluid, and chosen such that the chemical reaction continues to occurbetween the resin and hardener for at least a year so that an extent ofreaction between the resin and hardener can be reliably determined in alaboratory within the course of a year. In this example, the user of thepen 2 may insert a new mixing region 22/writing tip 24 into the pen 2,as shown in FIGS. 2 a and 2 b, thus abutting against through-holes 34,32, so as to invoke flow of fluids 8, 12 into the mixing region 22. Oncethe first and second fluids 8, 12 mix in the mixing region 22, theybegin to react and to solidify. The fluids 8, 12 continue to absorb andtravel downward through the mixing region 22 toward the writing tip 24,where they may be dispensed as a substantially homogeneously mixed fluidto a writing surface. Once they have begun to mix in the mixing region22, the user may then have around 24 hours in which to write using pen2. After those 24 hours, if he has not used the pen 2, the substantiallyhomogeneously mixed fluid in the mixing region 22 and/or writing tip 24will harden beyond the ability of the pen 2 to deliver the substantiallyhomogeneously mixture as a fluid. Then, in order to continue using thepen 2, the user should remove the used/hardened mixing region 22/writingtip 24 and replace it with a new mixing region 22/writing tip 24, wherethe process begins again.

After the user has used the pen 2 to write on the writing surface, theresin and hardener continue to react over the course of the next year ormore. Assume, now, that it is one year later, and the user must provethat he made the writing the previous year. He sends the writing to alaboratory for analysis, where the cured epoxy/ink is dissolved in asolvent. Depending on a rate or extent of dissolution of the ink (suchas a relative rate of dissolution, compared with an entire mass of curedink on the writing surface, which can be determined in ways well knownto those skilled in the art), and by knowing a relationship between theabsolute or relative rate of dissolution of the ink and a time of mixingof the resin and hardener, an approximate time/date of mixing of thefirst and second fluids 8, 12 can be determined. However, because thewriting could have been made by the user any time within the 24-hourwindow, the user will not be able to prove when, during that 24-hourperiod after the first and second fluids 8, 12 were mixed, that he madethe writing. Nevertheless, a very good approximation of the time ofwriting can be determined, and the invention is relatively impervious tofraud by pre-mixing. Epoxy resins having fast and slow rates of curingare well known in the art. Further, how to choose the proper resin andhardener to achieve the required rate of solidifying/curing, and theexact compounds and proportions thereof, is either well known in the artand/or can be determined with minimal experimentation, and will not bediscussed further. Further, other methods of determining an extent ofreaction between the first and second chemicals (in this example, theresin and hardener) will be known to those skilled in the art. Anotherexample is by measuring the color of the ink (e.g., doing a spectralanalysis of reflectance versus frequency), where the color of thesubstantially homogeneously mixed fluid changes in a known, predictableway with time.

It is important in this embodiment that the first and second chemicals(resin and hardener) are mixed at the proper ratio. A different mixingratio, as known by those skilled in the art, may result in a differentcuring rate. The curing rate is important for at least two reasons: a)first, the previously discussed window of time in which thesubstantially homogeneously mixed fluid may be deposited as ink must becontrolled (e.g., if it is too long, then the date-of-mixing analysisresults will be too fuzzy; if it is too short, then the user mustreplace the mixing region 22/writing tip 24 too often); b) second, ifthe reaction between the first and second chemicals, themselves, areused to date the substantially homogeneously mixed fluid, their rate ofreaction will affect the dating analysis. Thus, in the above embodiment,in which the second fluid contains hardener and a black due, the firstfluid may contain a resin and a blue (or otherwise differently colored)dye. Then, a resulting color (e.g., reflectance spectrum) of thesubstantially homogeneously mixed fluid may indicate relativeconcentrations of the first and second fluids in the substantiallyhomogeneously mixed fluid. Based on this information, the lab analystswho are charged with approximating the date that the first and secondfluids were mixed and deposited as the substantially homogeneously mixedfluid may adjust their analysis according to a knowledge of the reactionrate between the first and second chemicals at these concentrations.

Alternatively or in addition, the first fluid may contain a thirdchemical and the second fluid may contain a fourth chemical chemicallyreactive with the third chemical, such that the reaction between thethird and fourth chemicals has a known time dependence, and such that anextent of chemical reaction between the third and fourth chemicals isindicative of a time of mixing of the first and second fluids. Further,at least one of the third and fourth chemicals may have a color that ismeasurable by measuring a color of the deposited substantiallyhomogeneously mixed fluid, such that the color changes during chemicalreaction between the third and fourth chemicals, and such that a colorof the substantially homogeneously mixed fluid is indicative of a timeof mixing of the first and second fluids. Preferably, at least one ofthe third and fourth chemicals is entirely or substantially nonreactivewith air. Chemicals that react together in known, predictable ways arewell known in the art, including chemicals whose reaction results in achange of color, and will not be discussed further here, although someexamples follow. For example, the third chemical could contain at leastone oxygen supplying material which may be a metal oxide, an inorganicmetal or ammonium salt, or an oxygen-containing organic salt, and thefourth chemical could contain at least one metal or metalloid, such thatthe resulting mixture is capable of chemically reacting to yield heatand reaction by-products. For example, the third chemical could comprisean oxidizer (having a color), such as one of the metal oxides (e.g.,iron oxide), nitrates (e.g., potassium nitrate), chlorates,perchlorates, etc., and the fourth chemical could comprise a reducer,such as a metal (e.g., aluminum or iron). While the reducer may oxidizeby reaction with oxygen atoms in the atmosphere (and may thus besusceptible to the problems with the prior art discussed previously),the colored oxidizer (such as rust-colored iron oxide) will not. Thus,by measuring an extent of reaction of the oxidizer (e.g., by measuringthe color of the ink), the measurement could then be compared with aknown reaction rate of the oxidizer with the reducer to determine anapproximate time at which the first and second fluids were dispensed asthe substantially homogeneously mixed fluid. In order to prevent aslowdown of reaction by the oxidizer due to the reducer reacting withair, the substantially homogeneously mixed fluid may contain a greaterconcentration of reducer than is stochastically needed to react with thefluid's concentration of oxidizer. In another embodiment, the third orfourth chemical (e.g., the oxidizer or reducer) could be in a particularstructural form in the first or second fluids, such as in prill (orsubstantially spherical) form. The rate of reaction between the thirdand fourth chemicals would then depend on an average size and shape ofthe form, so that a rate of dissolution of the third or fourth chemical(during dating analysis) will depend on time and the average size andshape of the form of the third or fourth chemical in the substantiallyhomogeneously mixed fluid. Other means for regulating and subsequentlymeasuring the extent of reaction between the third and fourth chemicalswill be apparent to one skilled in the art, and are included in thescope of the present invention. For example, the third or fourthchemical may have a particular crystalline structure that may changeupon reaction with the other of the third or fourth chemical, and thischange may be measurable during the dating analysis. Also, the third andfourth chemicals, when mixed, may result in a solution such that the αand β phases of the mixture changes with time, as understood by oneskilled in the art.

Measuring the color of the substantially homogeneously mixed fluid mayinclude shining light with a wide range of frequencies (such as frominfrared to ultraviolet) and measuring the reflectance of the ink as afunction of frequency. The frequencies that the ink will absorb (and theextent of the absorption) may change in a known, predictable way overtime, due to reaction between the third and fourth chemicals (or betweenthe first and second chemicals, as discussed with regard to the previousembodiment).

In the above embodiments, where the time-indicating reaction occursbetween the third and fourth chemicals, the first and second chemicals(e.g., resin and hardener) still serve the purpose of preventing adefrauder from mixing the first and second fluids prematurely (i.e.,long before the actual writing will take place) and causing thetime-dependent chemical reaction between the third and fourth chemicalsto begin prematurely, because the defrauder will only be able to use theresulting substantially homogeneously mixed fluid during the previouslydiscussed window of opportunity (which may last only a few hours, up toa week or so).

In another embodiment, the third chemical may be the resin (e.g., thethird chemical could be the first chemical), and the fourth chemical maybe a chemical that is chemically reactive with the resin, such as anoxidizer. Similarly to the embodiment discussed above, the resin andoxidizer may react in a known, predictable way such that the extent oftheir reaction is indicative of a time that the first and second fluidswere substantially homogeneously mixed. A benefit to this embodiment isthat a third distinct chemical need not be added; the resin, hardener,and fourth chemical (where the first fluid contains the resin and thesecond fluid contains the hardener and fourth chemical) are sufficient.In another embodiment, because the writing surface to which thesubstantially homogeneously mixed fluid is dispensed is typically paperor a similar organic substance, the substantially homogeneously mixedfluid may contain only a third chemical that is chemically reactive withthe paper or writing surface. For example, the third chemical could bean oxidizer or an acid, either of which will react with the paper overtime. For example, the third chemical could comprise a very lowconcentration of nitric acid, which will slowly attack the paper. Anextent of reaction between the acid and paper could indicate a time ofmixing of the first and second fluids.

In many of the above embodiments, the chemical reaction between two ormore chemicals that indicates a time of mixing of the first and secondfluids may be artificially accelerated with heating, thus making thesubstantially homogeneously mixed fluid appear like it was dispensed tothe writing surface at a time before it actually was. This problem maybe assuaged by adding to at least one of the first and second fluids aheat activated indicator. Heat activated indicators are well known inthe art; examples are discussed in U.S. Pat. Nos. 4,531,141, 5,180,652,5,248,555, 5,368,905, 5,718,513, 6,030,118, 6,579,829, and 6,602,594.Heat activated indicators may comprise a single (possibly heterogeneous)substance, such as microcapsules that contain a heat-fusible substanceand a powdery dyestuff such that the color of the heat activatedindicator changes when the temperature increases, or may comprise two ormore separate substances such that, when mixed, become susceptible tocolor (or other) change upon application of heat or a temperature changeor increase. Heat activated indicators that change variably withvariable increases in temperature are known, as well as heat activatedindicators whose change is permanent. Thus, either the first or secondfluid may contain the heat activated indicator, or else the first fluidmay contain one component and the second fluid may contain anothercomponent of the heat activated indicator. A benefit to the secondaspect is that the first and second fluids, prior to being substantiallyhomogeneously mixed by the pen according to the present invention, maybe stored in a relatively hot place without affecting the ability of thepen to dispense a substantially homogeneously mixed fluid whose time ofmixing can be determined.

However, after the substantially homogeneously mixed fluid has beenmixed (or after the heat activated indicator becomes susceptible to atemperature increase, such as above a certain threshold), the color (orsome other measurable characteristic) of the substantially homogeneouslymixed fluid will change if the substantially homogeneously mixed fluidis heated too much. This change in color (or characteristic) willindicate to the laboratory analysts who have been charged withdetermining the date that the substantially homogeneously mixed fluidwas dispensed to the writing surface that the ink/fluid has been heated,and therefore that the results may not be accurate, because heating ofthe ink/fluid may artificially accelerate reaction in the ink,indicating an incorrect date of mixing of the first and second fluids.However, if the heat activated indicator has not been activated, thenthe analysts can be relatively certain that the ink/fluid has not beenexcessively heated (such as to above a threshold temperature), andtherefore the resulting analysis (of determining the date of mixing ofthe first and second fluids) is accurate.

For example, assume that the first fluid contains a heat activatedindicator that changes color (i.e., at least one aspect of itsreflectance spectrum changes in a clearly measurable way) at atemperature above 100° F., which is substantially above a roomtemperature at which documents are normally kept or stored. A user usesthe pen according to the present invention (such that the first fluidalso contains, e.g., a resin and a third chemical, and the second fluidcontains, e.g., a hardener and a fourth chemical chemically reactivewith the third chemical, as previously discussed) to make a writing. Shestores the writing in a file cabinet in her home, where the temperaturedoes not exceed 100° F., but on average is 76° F. A year later, she ischarged in a litigation with proving that she wrote the document atleast 6 months earlier (which happens to be 6 months after she actuallywrote it), so she sends the writing to a lab for analysis. At the lab,measurements (such as dissolution or optical measurements) are taken onthe ink/fluid, and the extent of chemical reaction between the third andfourth chemicals is determined. Because the heat activated indicator hasnot been activated, the lab analysts determine that the ink/fluid hasnot been subjected to temperatures higher than 100° F. Because no otherinformation is available, the analysts assume that the ink/fluidremained at 100° F. since the mixing of the first and second fluids.Based on a knowledge of a rate of reaction between the third and fourthchemicals at 100° F., the analysts conservatively estimate, for example,that the first and second fluids must have been mixed at some timebefore 10 months prior to analysis. (Of course, if the analysts couldassume the truth, which in this case is that the average temperature ofthe ink/fluid was 76° F., then their analysis would show that the firstand second fluids were mixed a year before analysis.) Thus, the userwill be able to prove what she is charged with proving.

On the other hand, if the user was a defrauder, who actually wrote thewriting yesterday, and who is charged with proving that she wrote thewriting more than six months ago in order to win a litigation, she willnot be able to do so. If she does not heat the ink/fluid, the analysiswill show that the first and second fluids were mixed yesterday. If shedoes heat the ink/fluid so as to accelerate the reaction between thethird and fourth chemicals, the heat activated indicator will indicate atemperature greater than 100° F., and the analysts will report so (andwill not be able to give a reliable estimate of the date of mixing thefirst and second fluids). Thus, the present invention preventsdefrauders from counterfeiting the date of a writing.

The fact that the color of many of the chemicals in the first and secondfluids must be subsequently or simultaneously measured to gain certaininformation (such as an extent of reaction between the third and fourthchemicals, such as a relative concentration of the first fluid in thesubstantially homogeneously mixed fluid, such as an activation of theheat activated indicator, etc.) is not a problem, because each piece ofinformation may be gleaned by looking at a different portion of a fullspectral reflectance analysis of the ink/fluid. For example, the thirdand fourth chemicals, when reacting, may result in a substantial changein the reddish region of the reflectance spectrum, while the heatactivated indicator, if activated, may result in a change in the UVregion of the reflectance spectrum, so that both pieces of informationcan be gained from a single spectrum reflectance analysis.

At least one of the first and second fluids may also comprise a chemicalidentifier, so as to be able to identify or determine some feature ofthe fluids. For example, the chemical identifier may be used to indicatea manufacturer of the fluids, so that the lab analysts can be morecertain that the fluids were not counterfeited by a defrauder. Forexample, the chemical identifier may comprise a rare-earth element, asknown by those skilled in the art, such as Europium (Eu) or Gadolinium(Gd). A presence of the chemical indicator may indicate that thesubstantially homogeneously mixed fluid is authentic, because the firstand second fluids were manufactured by a reputable manufacturer.Further, the substantially homogeneously mixed fluid may comprise asecond chemical identifier, such as a second rare-earth element, suchthat a relative concentration of the first rare-earth element to thesecond-rare earth element is a “key” that better and moreauthoritatively authenticates the substantially homogeneously mixedfluid. The first and second chemical identifiers (such as rare-earthelements) should be chosen so that their concentrations can be measuredaccurately and repeatably. For example, it is known in the art that bothEu and Gd are very sensitive to neutron activation analysis, in whichstable forms of Eu and Gd are nuclearly activated by placing them in aneutron field (such as from a nuclear reactor); the resulting activatedunstable isotopes of Eu and Gd can be measured using a quality gamma raydetector and the relative concentrations of Eu and Gd in the originalsample determined. In addition or alternatively to a chemical identifierused to identify and authenticate a manufacturer of the first and secondfluids, the chemical identifier may be used to identify a “model” or“type” of first and second fluids. For example, one combination of firstand second fluids may comprise chemicals chosen such that the resultingsubstantially homogeneously mixed fluid is most accurately datable in arange from one day to one year, and another combination of first andsecond fluids may comprise chemicals chosen such that the resultingsubstantially homogeneously mixed fluid is datable up to five years.Different chemical identifiers may be placed in each of these differentcombinations so that the lab analyst charged with dating the ink/fluidon the writing surface know which “chart” or equations to use in datingthe ink/fluid. As another example, one combination of first and secondfluids may comprise chemicals chosen such that the resultingsubstantially homogeneously mixed fluid is substantially blue, andanother combination of first and second fluids may comprise chemicalschosen such that the resulting substantially homogeneously mixed fluidis substantially red (because people like using different colored inks).Different chemical identifiers may be placed in each of these differentcombinations so that the lab analysts charged with dating the ink/fluidon the writing surface know which reflectance spectra or equations touse in dating the ink/fluid.

There are, of course, many other ways of mixing the first and secondfluids, and the present invention is not limited to the embodiment shownin FIGS. 1, 2 a, and 2 b. Another example will be shown with referenceto FIG. 3. In FIG. 3, a pen 102 according to another preferredembodiment of the present invention comprises a housing 104, a reservoir106 inside the housing 104 and configured to contain a fluid, a fluidink 108 contained in the reservoir 106, a writing tip 112, and a ballpoint 114 connected to the writing tip 112.

The fluid ink 108 comprises at least a first chemical, and a secondchemical (chemically reactive with the first chemical) encapsulated inmicrocapsules 110. Microcapsules are well known in the art; examples aredescribed in U.S. Pat. Nos. 5,589,523 and 6,506,494. The microcapsules110 are preferably very small, such as less than 20 μm, preferably lessthan 10 μm, and most preferably less than 1 μm. They have a wall made ofa substance that is not substantially chemically reactive with either ofthe first or second chemicals. The wall may be broken (and the secondchemical released into the first chemical) by a physical force appliedto the microcapsules 110, by heat, or by any other appropriate method.In a preferred embodiment, the pen 102 is a ball-point pen having ballpoint 114 that is configured to dispense the fluid ink 108 to a writingsurface by rolling with respect to the writing surface. The ball point114 comprises a hard metal, such as stainless steel, to which pressureis applied during the writing process. This pressure squeezes andultimately breaks or crushes the microcapsules 110, such as between theball point 114 and the writing surface. The second chemical releasedfrom the microcapsules 110 then substantially mixes with the firstchemical and a reaction between the first and second chemicals begins.Thus, the pen 102 and fluid ink 108 are configured so that the first andsecond chemicals are substantially chemically isolated from each other(i.e., they won't chemically react) prior to dispensing the fluid ink108 to the writing surface, and the first and second chemicals aresubstantially mixed with each other upon dispensing the fluid ink 108 tothe writing surface so as to chemically react with each other.

The first and second chemicals may, of course, be the resin and hardener(or any two chemicals that solidify into a solid product upon chemicallyreacting), respectively or vice versa, as discussed with respect to theembodiment shown in FIG. 1. Further, the fluid ink 108 preferablycontains at least one dye, so that the fluid ink 108 may be used as anink having contrast with a writing surface. Further, the fluid ink 108may contain a third chemical chemically reactive with the dye such thatchemical reaction results in a change in color, such that a color of thefluid ink on the writing surface is indicative of an extent of reactionbetween the third chemical and dye (which is indicative of a time ofdispensing of the fluid ink 108), and such that the third chemical (ordye, but not both) is encapsulated in microcapsules 110 so that thethird chemical and dye are not mixed until being dispensed by the pen102. Alternatively, the fluid inside the microcapsules 110 may beconsidered a second fluid, and the fluid outside the microcapsules 110may be considered a first fluid (or vice versa), and the embodimentspreviously discussed with respect to the chemical compositions of firstand second fluids may also apply to this embodiment. For example, thefirst fluid may contain a heat activated indicator, a resin, a thirdchemical, and a first dye, and the second fluid may contain a chemicalidentifier, a hardener, a fourth chemical, and a second dye.

There are, of course, other ways that the first and second fluids offluid ink 108, separated in the reservoir 106 via the walls ofmicrocapsules 110, may be mixed upon dispensing the fluid ink 108.Further, first and second fluids could be contained in separatereservoirs prior to using the pen, and then mixed when the pen is readyto used. For example, a first reservoir could be made of a breakable oropenable or rupturable substance and could contain the first fluid, andthe second reservoir could be made of a malleable substance and couldcontain the second fluid and the first reservoir. When the user is readyto use the pen, he can simply squeeze the second reservoir to break orrupture the first reservoir inside the second reservoir, causing thefirst fluid to flow into and substantially homogeneously mix with thesecond fluid. Then, the substantially homogeneously mixed fluid could bedispensed to a writing surface, as discussed previously. Because thefirst and second fluids will begin to solidify, the user has the windowof opportunity in which to use the pen, after which the pen may bedisposed of. Further, if the first fluid contains a first dye and thesecond fluid contains a second different dye, the extent of homogeneityof the mixed fluid can be determined by the lab analysts by doing aspectral reflectance analysis; if the mixed fluid is not substantiallyhomogeneously mixed, then they may decide that the dating analysis isnot valid.

1. A pen, comprising: a housing; a first reservoir inside said housingand configured to contain a first fluid, said first reservoir comprisinga first opening configured to dispense said first fluid; a secondreservoir inside said housing and configured to contain a second fluid,said second reservoir comprising a second opening configured to dispensesaid second fluid; a first writing tip extending from said housing andconfigured to dispense a substantially homogeneously mixed fluid to awriting surface; a mixing region connected to said first and secondopenings and said first writing tip and configured to substantiallyhomogeneously mix said first and second fluids and dispense saidsubstantially homogeneously mixed fluid to said first writing tip; and asecond writing tip extendable from said housing and configured toreceive only said first fluid and to dispense said first fluid unmixedwith said second fluid to said writing surface.
 2. A pen, comprising: ahousing; a first reservoir inside said housing and configured to containa first fluid, said first reservoir comprising a first openingconfigured to dispense said first fluid; a second reservoir inside saidhousing and configured to contain a second fluid, said second reservoircomprising a second opening configured to dispense said second fluid; afirst writing tip extending from said housing and configured to dispensea substantially homogeneously mixed fluid to a writing surface; a mixingregion connected to said first and second openings and said firstwriting tip and configured to substantially homogeneously mix said firstand second fluids and dispense said substantially homogeneously mixedfluid to said first writing tip; and said first and second fluids,wherein said first fluid comprises a first chemical and said secondfluid comprises a second chemical that is chemically reactive with saidfirst chemical.
 3. The pen as in claim 2, wherein said first and secondchemicals solidify into a solid product upon chemically reacting.
 4. Thepen as in claim 3, wherein said first fluid further comprises a firstdye.
 5. The pen as in claim 4, wherein said second fluid furthercomprises a second dye, and wherein a color of said substantiallyhomogeneously mixed fluid is indicative of a concentration of said firstfluid in said substantially homogeneously mixed fluid.
 6. The pen as inclaim 3, wherein said first and second chemicals solidify into a solidproduct at a rate such that a viscosity of said substantiallyhomogeneously mixed fluid at 25 degrees Celsius is greater than 10,000centipoise at any time after seven days after a time of mixing of saidfirst and second fluids, wherein said viscosity is less than 10,000centipoise at any time before one day after the time of mixing of saidfirst and second fluids.
 7. The pen as in claim 3, wherein said firstfluid further comprises a third chemical and said second fluid comprisesa fourth chemical chemically reactive with said third chemical, whereinthe chemical reaction between said third and fourth chemicals has aknown time dependence, and wherein an extent of chemical reactionbetween said third and fourth chemicals is indicative of a time ofmixing of said first and second fluids.
 8. The pen as in claim 3,wherein said first fluid further comprises a first chemical identifier,said first chemical identifier comprising at least a first rare-earthelement, and wherein a relative concentration of said first rare-earthelement in said fluid ink is indicative of a manufacturer of said fluidink.
 9. The pen as in claim 2, wherein said mixing region and firstwriting tip are configured to be removable from said pen and readilyreplaceable.
 10. An ink pen for dispensing ink having time-dependentcharacteristics, comprising: a writing pen, comprising at least onereservoir configured to contain a fluid, and further comprising awriting tip configured to dispense said fluid to a writing surface; anda fluid ink having time-dependent characteristics contained in said atleast one reservoir, said ink comprising: a first chemical; a secondchemical; and a dye, wherein said first and second chemicals arechemically reactive and wherein their chemical reaction causes them tosolidify into a solid product, wherein said writing pen and fluid inkare configured so that: at least said first and second chemicals aresubstantially chemically isolated from each other prior to dispensingsaid ink to said writing surface; and at least said first and secondchemicals are substantially mixed with each other upon dispensing saidfluid ink to said writing surface so as to chemically react with eachother.
 11. The ink pen as in claim 10, wherein said fluid ink iscontained in exactly one reservoir, and wherein at least one of saidfirst and second chemicals is encapsulated in microcapsules.
 12. The inkpen as in claim 10, wherein the chemical reaction between said first andsecond chemicals has a known time dependence, and wherein an extent ofchemical reaction between said first and second chemicals is indicativeof a time of dispensing of said fluid ink to said writing surface. 13.The ink pen as in claim 10, wherein said fluid ink further comprises aheat-activated indicator.
 14. The ink pen as in claim 10, wherein saidfluid ink further comprises at least a third chemical chemicallyreactive with said dye such that chemical reaction results in change ofcolor, wherein said writing pen and fluid ink are configured so that: atleast said dye and said third chemical are substantially chemicallyisolated from each other prior to dispensing said ink to said writingsurface; and at least said dye and said first chemical are substantiallymixed with each other upon dispensing said fluid ink to said writingsurface so as to chemically react with each other, wherein a color ofsaid fluid ink is indicative of an extent of chemical reaction by saiddye, and wherein an extent of chemical reaction by said dye isindicative of a time of dispensing of said fluid ink to said writingsurface.
 15. The ink pen as in claim 10, wherein said fluid ink furthercomprises third and fourth chemicals chemically reactive with eachother, wherein said writing pen and fluid ink are configured so that: atleast said third and fourth chemicals are substantially chemicallyisolated from each other prior to dispensing said ink to said writingsurface; and at least said third and fourth chemicals are substantiallymixed with each other upon dispensing said fluid ink to said writingsurface so as to chemically react with each other, and wherein an extentof chemical reaction between said third and fourth chemicals isindicative of a time of dispensing of said fluid ink to said writingsurface.
 16. The pen as in claim 2, wherein the chemical reactionbetween said first and second chemicals has a known time dependence, andwherein an extent of chemical reaction between said first and secondchemicals is indicative of a time of mixing of said first and secondfluids.
 17. The pen as in claim 2, wherein said first fluid comprises athird chemical chemically reactive with a paper writing surface, whereinan extent of chemical reaction between said third chemical and saidpaper writing surface is indicative of a time of mixing of said firstand second fluids.
 18. The pen as in claim 2, wherein said substantiallyhomogeneously mixed fluid comprises a heat-activated indicator.
 19. Amethod of dating a writing, comprising: providing a writing created bythe pen as in claim 2; measuring an extent of interaction between thefirst and second fluids; and determining an approximate time of creationof the writing based at least in part on the measuring.
 20. A method ofcreating a time-stamped writing, comprising: providing the pen as inclaim 2; and creating a writing on a writing surface using the pen,wherein an approximate time of the creating is ascertainable bydetermining an extent of interaction between the first and secondfluids.